A range of specialized foods (meal replacers and/or meal supplements) exist for elderly or convalescents or other patients that cannot get the nutrition required by eating normal foods or are unable to feed themselves or require assistance during feeding. Generic terms used to categorise these foods include “medical foods”, “enteral foods”, “enteral nutrition”, “medical liquids”, and the like, and are collectively used to refer to foods that are taken under the supervision of a medical professional. In some jurisdictions medical foods/enteral nutrition has a legal definition. In the USA, the term medical food, as defined in section 5(b) of the Orphan Drug Act (21 U.S.C. 360ee (b) (3)) is “a food which is formulated to be consumed or administered enterally under the supervision of a physician and which is intended for the specific dietary management of a disease or condition for which distinctive nutritional requirements, based on recognized scientific principles, are established by medical evaluation”. In some jurisdictions, such foods are available to the public only by prescription, in others they can be procured directly over the counter (OTC).
Enteral formulas are ingested both orally and through tubes. Oral ingestion is useful when nutrient supplements are necessary and both the digestive tract and the patient are capable of taking them. Tube feeding is necessary for patients who need supplements but cannot take nutrition orally.
All these foods have very exacting requirements. They require a high degree of heat treatment to provide sterility and long shelf life stability, high calorific density, i.e. highly concentrated doses of nutrients, but at the same time low viscosity so that they can be readily administered to the patient and consumed easily. In order to obtain a long shelf life of the liquid compositions sterilization is the preferred heat treatment, in particular ultra high temperature (UHT), wherein the product is heated (indirectly by means of heating coils or directly by live steam under pressure) at 135-150° C. and held at this temperature for 4-10 seconds, followed by aseptic packaging. Another possibility is the so called retort process, wherein the product is sterilized by sealing in cans which are then heated in an autoclave at 110-130° C. for 10-20 minutes.
Liquid nutritional foods are also used by healthy subjects as meal replacers or when a rapidly consumable feed is required. Liquid nutritional foods are generally suitable for use by children, the aged or by athletes and for these consumers the organoleptic properties of the product such as, for instance viscosity, mouthfeel, smell and colour are very important.
Whey protein is recognised as a suitable protein source to treat persons suffering from diseases or conditions or as a result of treatment for a disease or condition, such as from cachexia, sarcopenia, as well as a valuable source of nutrition for healthy persons, such as sportsmen and active elderly. As a source of whey protein to be used in the present invention, any commercially available whey protein source may be used, i.e. whey obtained by any process for the preparation of whey known in the art, as well as whey protein fractions prepared thereof, or the proteins that constitute the bulk of the whey proteins being β-lactoglobulin, α-lactalbumin and serum albumin, such as liquid whey, or whey in powder form, such as whey protein isolate (WPI) or whey protein concentrate (WPC). However, it will be appreciated that whey protein or whey protein fractions without suitable processing will typically form a gel when heated under certain conditions (heating above pH 6.5 results in firm elastic gels; whilst coagula are formed below pH 6.5) and that the formation of a gel is detrimental to formulation of the heat stable liquid nutritional compositions.
It has been reported that the heat stability of whey protein-stabilised emulsions is particularly sensitive to pH and ionic strength (Demetriades K & McClements D J (1998), Influence of pH and heating on physicochemical properties of whey protein-stabilized emulsions containing a non-ionic surfactant. Journal of Agricultural and Food Chemistry, 46, 3936-3942; Demetriades K, Coupland J N & McClements D J (1997), Physical properties of whey protein stabilized emulsions as related to pH and NaCl. Journal of Food Science, 62, 342-347; Hunt J A & Dalgleish D G (1995), Heat stability of oil-in-water emulsions containing milk proteins: effect of ionic strength and pH. Journal of Food Science, 60, 1120-1123). At a pH near the isoelectric point (pI) of the proteins the charge on the protein-containing droplets is low, and therefore protein-protein interactions are favoured and protein aggregation occurs rapidly.
However, when the pH is adjusted away from the pI (weighted average of the principal whey proteins is pH 5.0), charges on the protein molecules are increased. Greater electrostatic repulsive forces must be overcome for aggregation to occur, and therefore the rate of aggregation is slowed. Previously, in order to produce a low viscosity emulsion with a long shelf-life, whey proteins could only be used when the pH of the system was sufficiently distant from the isoelectric point of the whey proteins, i.e. <pH 4 or >pH 6, to avoid the formation of a high-viscosity liquid, paste or gel.
It is also reported that the presence of divalent (i.e. calcium and magnesium) and/or monovalent (i.e. sodium, potassium) cations adversely affect the physicochemical properties and stability of protein stabilised emulsions (Keowmaneechai E & McClements D J (2002), Effect of CaCl2 and KCl on physiochemical properties of model nutritional beverages based on whey protein stabilized oil-in-water emulsions. Journal of Food Science, 67, 665-671; Kulmyrzaev A A, & Schubert H (2004), Influence of KCl on the physicochemical properties of whey protein stabilized emulsions. Food Hydrocolloids, 18, 13-19; Ye A & Singh H (2000), Influence of calcium chloride addition on the properties of emulsions stabilized by whey protein concentrate. Food Hydrocolloids, 14, 337-346). Increasing the ionic strength of the aqueous phase by adding minerals can cause electrostatic screening of the charges on the proteins, which leads to decreased electrostatic repulsion between droplets and thus promotes aggregation. Divalent ions can have more pronounced effects than monovalent ions. As will be appreciated by those skilled in the art, the higher the protein concentrations in the composition, lower quantities of minerals would be sufficient to generate the adverse effect.
It is well known in the prior art that high temperature processing can lead to the generation of a sulphurous off-flavour in whey protein containing emulsions such as milk (Steely J S (1994) Chemiluminescence detection of sulphur compounds in cooked milk. In: Sulfur compounds in Foods, ACS Symposium Series 564). pH has been shown to have a significant effect on the heat-activated sulfhydryl (—SH) groups of skim milk whey that evolves during heating to at least 90° C. As the pH of the whey is lowered below pH 6.0, the quantity of sulphides evolved is decreased. In contrast, an increase in pH above 6 to about pH 9 is accompanied by an increase in the amount of heat volatile sulphides (Townley R C & Gould I A (1943), A quantitative study of the heat labile sulfides of milk. III. Influence of pH, added compounds, homogenization and sunlight Journal of Dairy Science, 26, 853-867). In spite of the flavour benefits at pHs below 6.0, it is well known in the prior art that decreasing the pH of the composition to around the isoelectric point would lead to limited processability since whey proteins are prone to aggregation in this pH range.
It is an object of the invention to overcome these difficulties and to provide high protein liquid nutritional compositions having desirable pH and heat stability, or to provide the public with a useful choice.